Various optical pump sources such as flashlamps have been used to excite gain media since the initial development of the laser. Light energy from the pump source excites the lasing species in the gain medium to the upper energy states allowing coherent light energy to be amplified.
The subject invention was developed for use with a flashlamp pumped, high power, high repetition rate phase conjugate laser amplifier. In this system, the pulsed output from a low power, master oscillator is directed through a flashlamp excited amplifier. This system is designed not only to produce high power pulses at high repetition rates, but also to produce a near diffraction limited beam. This laser is being marketed under the trademark Infinity by the assignee herein and is described in greater detail in copending U.S. patent application, Ser. No. 08/196,411, filed Feb. 15, 1994.
This laser system has a number of different applications such as laser pumping laser oscillators and OPO's, lidar, pulsed holography, machining, lithography and photoablation. Many of these applications require high output power pulses with good beam quality. Many of these applications also require that the energy of each pulse remain relatively constant. One problem with achieving this goal is that the total light generated by a flashlamp during an output pulse can vary significantly from pulse to pulse even if the same initial conditions are applied to the flashlamp.
The reasons for this variation in the light generated from the flashlamp are complex and not well understood. However, part of the problem appears to relate to the fact that the resistance characteristics of the flashlamp are not consistent from one shot to the next which may be due to the wandering of the arc in the flashlamp. In practice, it was found that the light energy out of even a relatively stable flashlamp could vary by about one percent (peak to peak) at low repetition rates and rise to a level of three percent at higher repetition rates. Although these variations may appear small, they can result in the introduction of significant noise or jitter into the output of the system which can be equal to or several times greater than the variations in the flashlamp output.
In the prior art, various techniques have been developed to control the amount of light generated by a flashlamp. In these techniques, the output of the flashlamp is monitored during the pulse by a photodetector. The flashlamp control circuit will continue to supply excitation energy to the flashlamp until the desired total level of light output (as measured by the photodetector and integrated by a capacitor) has been obtained.
This approach is effective to control the level of the light output by the flashlamp. Unfortunately, this approach is not suitable for use with the laser system described above. More specifically, in the prior art flashlamp control technique , in order to control the light generated by the flashlamp, the length of the flashlamp pulse was allowed to vary. However, in order to be useful in many of the applications referred to above, the length of each flashlamp pulse must be the same so that the output can be synchronized to other instrumentation. Therefore, it is not possible to utilize a technique for controlling the light generated by a flashlamp which permits the length of the lamp pulses to vary.
Accordingly, it is an object of the subject invention to provide a flashlamp control technique which minimizes the variation in the light output of a flashlamp without varying the duration of the flashlamp pulses.